We all have that one friend who is intermittent fasting—and you probably hear about it, a lot. Intermittent fasting as a wellness trend has blown up in the last few years, mainly due to its claims that it can, in some way, change your life. You’ll have more energy, a clearer mind, and maybe even lose a few unwanted pounds. It’s almost too good to be true, and yet we continue to hear IF success stories, so it’s clear that it has some something there. (Even though truthfully, it’s never been my thing.)

In the midst of the debates around intermittent fasting is one question that always comes up: Is it healthy (or even safe) to fast before your workout? The opinions are widespread, so here’s a breakdown of the facts.

As you probably guessed, the answer is “it depends.”

There are many sources out there that will tell you that working out in a fasted state is the best way to lose weight, rev your metabolism, and improve your performance during your workout. While those outcomes can be true for some—it depends on who we’re talking about and the makeup of their body.

Unfortunately, most conversations about intermittent fasting focus on the benefits, not the potential drawbacks. And one unsavory (often overlooked) side effect of working out fasted is that you can end up losing muscle, not just fat.

“It is true that you may lose more fat working out fasted, however, you may also lose more muscle,” says Jaime Schehr, N.D., R.D. “If the body’s glycogen stores (aka energy stores) are depleted, the body can break down protein for fuel—the opposite of what most people are looking for.”

In other words, if you don’t have fuel in your tank because you’ve been fasting for so many hours, your glycogen stores will be empty, and your body could look to your muscles for sustenance.

Why should you care?

Well, our muscles are responsible for our movement, strength, and many other aspects of our overall health, but aside from that, they play a huge role in the efficacy of our metabolism. The more lean muscle you have, the faster your metabolism is, so if you’re losing muscle—you get the picture. Muscle breakdown is not the result you want from fasting, so if you’re concerned about it, consult with a medical professional (like a general or holistic doctor) before becoming a fasted-workout devotee.

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If you do decide to work out fasted…

...just make sure to adjust your eating window to allow for a post-workout meal. Specifically, a protein-filled post-workout meal, as consuming protein after exercising helps prevent muscle breakdown as well as injury.